The headphone jack on my MacBook Pro stopped working. It was working fine, then just spontaneously stopped while I was using external speakers plugged into the output jack. Now, neither speakers nor 3 different headphones have sound. The internal speaker on my MacBook Pro still works.

What other steps have you taken to try and get it working again? What OS? Have you tried restarting? Have you checked settings under System Preferences -> Sound?
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ephsmithJul 1 '12 at 17:26

Did the sound stop working while the external speakers were still plugged on or afterwards (i.e. when you plugged in the headphones)? In the second case there may be some lint in the jack.
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patrix♦Jul 1 '12 at 18:01

Were you using the computer at the time it stopped working? If so, what were you doing? What operating system version are you running? What is the volume set for when speakers are plugged in?
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Daniel♦Jul 1 '12 at 18:21

Shari USA, welcome to Ask Different! Please answer these additional questions and give us some more information, and we will be glad to help you.
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Wheat WilliamsJul 1 '12 at 20:40

1

Which model MacBook Pro do you have? Some of them have combination audio ports that can switch to optical audio. If optical audio is erroneously engaged, then your headphones (or any other non-optical hardware) won't work. You can switch it back if this is the case.
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iynqueJul 2 '12 at 4:22

I had this same problem with my MacBook Pro 13" (mid-2009) running OS 10.6.8. Somewhere on the internet I found the answer. After plugging in the headphones and looking at the System Preferences/Sound, you may not see (in theis model) the option for Headphones, only for Internal Speaker. (Of course, you must first check that the audio port is selected for output, since this Mac has only a single port audio. Also, that the mute is not selected.)
After seeing no Headphone option, put the Mac to sleep and wake it up again. Voila! Your headphones/external speakers will work.

But why?
To my understanding the reason is that Apple attaches the sound-on-or-off-state to the current output device.

So switching off the sound while having plugged in the headphones makes the headphones be silent. Plug them off and you will hear the internal speakers. Plug them in again (after a minute or after half a year) and they still will be silent. Because YOU switched off the sound OF THE HEADPHONES (some months ago).

(It is the same the other way round. But in that case you probably would try the F10 button ...)

(Note: you can see this behaviour also with the checked or unchecked mute button in System Preferences > Sound > Output, as mentioned in a post above. Its state depends on the selected output device.)

I'm not sure if it's a hardware issue for you; if you go under System Preferences → Sound → Output, you should see Headphones as one of the output devices. Most likely the audio is being directed to some other output device (like Soundflower or Boom). Just make sure that Headphones is selected.